1. Field of the Invention
The present invention relates to an alignment adjusting system for use in an optical system of an optical transceiver, in particular, which is used for intersatellite communications.
2. Description of the Related Art
Conventionally, optical communications using a beam of laser light have the following features:
(a) since the wavelength of the laser light is smaller than one thousandth that of the microwave, antennas and transceivers can be downsized;
(b) since a wider frequency band can be used, communications with a lager capacity can be performed in a higher speed; and
(c) since sharp laser beams are used, mutual interference among them can be neglected.
However, to make the best use of the above features, it is necessary to direct sharper laser beam to an antenna of the destination station with higher accuracy. For example, in an optical communication between a geostationary satellite and a low-earth-orbiting satellite, it is necessary to provide an optical antenna having an aperture diameter aperture of about 20 cm. When a conventionally used semiconductor laser light with a wavelength of 0.83 .mu.m is used, the half-width of a beam of transmitting laser light becomes approximately 5 micro-radian. In this case, pointing and tracking with an accuracy of 1 micro-radian or less is required to maintain a stable optical communication. Further, for an optical transceiver to be provided in a low-earth-orbiting satellite, it is necessary to provide a wider directivity range over 2.pi. steradian or more in solid angle in the sphere.
In a conventional pointing and tracking system, in order to simultaneously obtain a wider angle range and a higher tracking accuracy, there has been used a control system having a double feedback loop, comprising:
(a) a coarse tracking system having a relatively slow response speed but having a wider field of view, wherein the coarse tracking system comprises a CCD sensor, and two-axes gimbals for controlling an optical antenna with two directions; and
(b) a fine tracking system having a narrower field of view but having a higher speed response, wherein the fine tracking system comprises a quadrant detector sensor (hereinafter, referred to as a QD sensor) composed of four photodiode cells divided into four quadrants, and a fine pointing mirror module (hereinafter, referred to as an FPM).
Besides, a conventional center-fed cassegrain type optical antenna comprises a main mirror consisting of a concave parabolic mirror, and a submirror consisting of a convex hyperbolic mirror. In the center-fed cassegrain type optical antenna, after a beam of transmitting laser light outputted from an optical transceiver is led to the submirror through a hole formed in the center of the main mirror, the beam of laser light is reflected by the submirror, and then is reflected by the main mirror. Thereafter, the beam of transmitting laser light is transmitted to an optical antenna of a destination station.
Further, in a conventional pointing, acquisition and tracking control system, the initial acquisition is performed by receiving a beacon light having a wider field of view transmitted from a satellite of a destination station. In this case, a CCD sensor is used as the acquisition sensor, and then an error from the tracking center of the luminescent spot of the beacon light which is outputted from the CCD sensor is detected. Thereafter, based on the detected error, the two-axes gimbals mechanically connected with the optical antenna are controlled to be driven, so that the optical antenna is directed to the satellite of the destination station. The beacon light is then captured within the field of view of the QD sensor, and thereafter, tracking errors in two directions perpendicular to each other are detected based on a relative ratio of amounts of light incident on the respective photodiode cells of the QD sensor. Based on the tracking errors, the driving mechanism of the FPM is controlled to be driven so that a beam of laser light transmitted from the satellite of the destination station is captured within the field of view of a receiving photodiode sensor (hereinafter, photodiode sensor will be referred to as a PD sensor). Further, a point-ahead mirror module (hereinafter, referred to as a PAM) is used to correct a point-ahead angle of a beam of transmitting laser light so as to be substantially zero, namely, to accurately illuminate the destination station, which will be described in detail later. Then, a beam of laser light is transmitted in the corrected direction toward the optical antenna of the destination station, and then the optical communication is started.
To maintain the tracking accuracy under severe environments such as a large vibration at the launching of the satellite, a large variation in the ambient temperature, so that optical communication can be carried out, it is necessary to provide 1 .mu.m or less alignment accuracy for arrangement of devices of the optical system including mirrors and lenses. For this reason, conventionally, by using Invar (trademark: Ni-based alloy) or Zerodur (trademark: glass ceramic material), each of which has a small thermal expansion coefficient and has established performance results as a material constituting precision optical equipment, such design has been implemented that arrangement of optical devices will not change due to change in temperature.
However, there have been such problems that the above-mentioned Invar has a relatively large specific gravity while the above-mentioned Zerodur encounters difficulty in processing the same into a complex shape.
Suited for structural materials for optical transceiver to be provided in the satellite are light-weight metal materials with a specific gravity as small as possible and a thermal conductivity as large as possible, such as Al, Mg, Be or the like. However, the light-weight metal materials, in general, have relatively large thermal expansion coefficients, respectively, and therefore, it is necessary to provide an alignment adjustment mechanism for compensating for alignment errors in equipment arrangement due to thermal expansion in order to achieve optical communications. However, there has been provided no proper method for adjusting alignment of the optical transceiver after launching of the satellite. Even if such adjustment is made possible in some way, severe demand for accuracy would cause the optical transceiver provided comprising an optical antenna to be relatively large sized because of the provision of the alignment adjustment mechanism, disadvantageously.